Bumping gaia.json for 1 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/3d38c67a7874
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 906316 - Don't download the xulrunner SDK each time we change a branch if their configuration are different r=yurenju

This also shows a message when the archive file seems to be corrupted, advising
to run "make really-clean". Also changed "make really-clean" to remove all
xulrunner related files.

We now uncompress the archive directly in a specific directory that should
change when we change the URL.

We use a dot file inside the xulrunner SDK directory to keep the URL used to
download that SDK.

This _will_ make all devs redownload XULRunner, sorry.